The removal of unwanted hair growth from the body can be accomplished with mechanized means, for example razors, tweezers or wax, all of which are uncomfortable to use, irritate the skin and/or cause damage to the skin. Another form of hair removal is by heating the hair growth to a temperature sufficient to cut the hair, however a concern of devices for hair removal involving heat is the danger of skin damage from excess heat. U.S. Pat. No. 6,825,445, issued Nov. 30, 2004 to Shalev et al., the entire contents of which is incorporated herein by reference, is addressed to an electric shaver comprising a heat generator and one or more heat elements heated to a temperature sufficient to cut hair, the heat generator arranged to prevent heat from being applied continuously in a single area for sufficient time to cause skin damage.
U.S. Pat. No. 7,170,034, issued Jan. 30, 2007 to Shalev et al., the entire contents of which is incorporated herein by reference, is addressed to an electric shaver comprising a heat element heated to a temperature sufficient to cut hair, the heating of the heat element being pulsed to prevent heat from being applied continuously in a single area for sufficient time to cause skin damage.
U.S. Pat. No. 7,202,446, issued Apr. 10, 2007 to Shalev et al., the entire contents of which is incorporated herein by reference, is addressed to an electric shaver comprising an elongate element heated to a temperature capable of cutting hair and a vibrating structure on which the elongate element is mounted, the vibrating structure arranged to prevent skin damage.
U.S. published patent application S/N 2009/0205208 published Aug. 20, 2009 to Shalev, et al, the entire contents of which is incorporated herein by reference, is addressed to a hair cutting device comprising a detector adapted to detect motion of the shaver heated wire arranged to cut hair, a hair cutting removal and suppression head having a heated wire suitable for heating hair growing from the skin and cutting the hair, and a controller arranged to move the hair cutting removal and suppression head between a hair cutting position and a retracted position responsive to the presence of, or absence of, detected motion.
It is known that heating hair follicles affects hair growth rate. Experience has shown that repeated use of heat based hair removal devices, such as certain no!no!® products available commercially from Radiancy, Inc. of Orangeburg, N.Y., substantially reduces hair growth rate. Although hair growth rate is reduced by the above mentioned products, hair growth rate reduction is achieved as a by product, and is thus not optimal.
In an article by G. B. Altshuler, et al, published in 2001 in Lasers in Surgery and Medicine, a theory of selective photothermolysis and an extended theory of selective photothermolysis are described. According to the theory of selective photothermolysis, irradiating the skin with electromagnetic radiation (EMR) of an appropriate wavelength and pulse time results in permanent thermal damage of targeted structures while leaving surrounding tissues intact. As applied to hair follicles, the pulse width of the supplied EMR is to be much smaller than the thermal relaxation time of the hair follicles, thus resulting in heat generated within the hair follicles not flowing out until they become fully damaged, while minimizing damage to the surrounding epidermis.
The extended theory of selective photothermolysis indicates that the EMR wavelength should be chosen to maximize contrast between the absorption coefficient of a pigmented area and that of tissue surrounding the target. Additionally, the EMR power should be limited to prevent absorption loss in the pigmented area, while ensuring that it is sufficient to achieve a temperature of the pigmented area higher than the target damage temperature. Finally, the pulse width should be made shorter, or equal to, the thermal damage time (TDT), where TDT is defined as the time required for irreversible target damage with sparing of the surrounding tissue. It is noted that the TDT may be significantly longer that the TRT of the entire target.
It would be desirable to adapt at least some of the teachings of selective photothermolysis to improve the results of hair cutting and shaving devices.